This invention relates to an optical-electrical measuring method, in which cross-sectional dimensions particularly of elongate articles are determined in geometric analogy to the known operation of mechanical depth indicators in that the distance from a point to be measured at the periphery of a cross-section to a straight reference line is determined, which contacts the periphery of the cross-section at at least two spaced apart points. By a pivotal movement of the entire measuring system a light beam, which is moved in the measuring field by a parallel displacement, is caused to contact in a measurable position assumed during that movement the periphery of the cross-section at said at least two points which determine the straight reference line when the measuring system is in a detectable angular position, and the distance from the point to be measured to the axis of the light beam in that contacting position is determined.
A method of that kind has been proposed in Published German Application 32 19 389, which is incorporated herein by reference and an apparatus for carrying out the method has also been described there. By means of that apparatus it is possible, e.g., to measure the movement of the light beam during its parallel displacement through the measuring field from its entrance into the region which is vignetted by the article disposed in the measuring field to its re-exit from that region when the entire measuring system is in such an angular position that the light beam exiting from the vignetted region contacts the periphery of the cross-section at at least two points, which determine a straight reference line. In that manner it is possible to measure at an angle section the width of each leg as the perpendicular distance from its terminal generatrix to the outside surface of the respective other leg or the height of a tee section. But that known measuring method cannot be used for measurements of the above-mentioned kind unless the point to be measured and those points on the periphery of the cross-section which determine the straight reference line constitute the boundaries of the vignetted region.
But for a complete measurement of angle, tee, C or I sections or similar elongate products it is necessary also to measure leg thicknesses, flange thicknesses, web thicknesses and flange depths in cases in which terminal points of a distance to be measured or points of the straight reference line lie in all angular positions of the measuring system at such locations that they are covered by the shadow cast by other parts of the periphery of the cross-section in parallel or parallel-scanning light so that they do not constitute the boundaries of the vignetted region. For this reason such sections cannot completely be measured by means of the known method.